Pre-incubation of guinea-pig myenteric plexus with β-funaltrexamine: discrepancy between binding assays and bioassays

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Abstract
1 The acute effects of β-funaltrexamine and the effects of pre-incubation with this compound were examined in five in vitro assay tissues and in selective binding assays in homogenates of guinea-pig brain and myenteric plexus. 2 In competitive displacement assays with selective ligands, β-funaltrexamine had highest affinity for the μ-binding site in the myenteric plexus and brain of guinea-pig. Its affinity for the k-site was about 15% of that for the μ-site. 3 Pre-incubation of the assay tissues with β-funaltrexamine caused an increase in the IC50 values of μ-and δ-receptor agonists but not of k-agonists. Although in bioassays on the myenteric plexus-longitudinal muscle preparation of the guinea-pig, the IC50 value of the μ-receptor ligand [D-Ala2, MePhe4, Gly-ol5] enkephalin was increased up to 124 fold, its binding at the μ-site in homogenates of the preparation was not affected by this treatment. 4 These findings indicate that the effects of pre-incubation with β-funaltrexamine on agonist potency of the μ-receptor ligand are due to an interference with the coupling mechanism between the μ-binding site and the effector system.